Depths
Bottom
noun
Something is in the interior AND the bottom? Why, it's in the very depths! And if it's in the interior and bottom of you, then it's at the bottom of your heart!
おくそこ
The readings for this word are both kun'yomi readings. You learned them when you learned the kanji, so you should be able to read this on your own.
心の奥底でほくそ笑みました。
I chuckled from the depths of my heart.